In the past, multimode optical fibers are employed primarily for short distance or low data rate communications. High data rate communications primarily employ single mode optical fibers. A representative optical fiber has a core surrounded by at least one cladding. When the optical fiber is packaged into an optical fiber cable, it may have a jacket or a coating for protecting the core and the cladding. Because the jacket and the coating are not related to the function of the present invention, they will not be included in the specification. One skilled in the art understands that the jacket or the coating may have to be removed before processing an optical fiber cable. For International Telecommunication Union (ITU) data communication and telecommunication applications, a representative industry standard single mode optical fiber has a core diameter or a mode field diameter in the neighborhood of 9 μm and a cladding outer diameter in the neighborhood of 125 μm, and a representative industry standard multimode mode optical fiber has a core diameter in the neighborhood of 50 μm to 62.5 μm and a cladding outer diameter in the neighborhood of 125 μm. These representative industrial standard optical fibers are made from silica. The ITU industry standard single mode optical fiber is suitable for single mode operations and supports a single propagation mode for light of wavelengths defined by ITU for data communication and telecommunication systems. The industry standard multimode optical fiber is suitable for multimode operations and supports multiple propagation modes for light of wavelengths defined by ITU for data communication and telecommunication systems. Light generally propagates through a multimode optical fiber at different speeds in different propagation modes. Consequently, light disperses over a relatively short distance when propagating in multiple propagation modes through a multimode optical fiber when compared to propagating in a single propagation mode through a single mode optical fiber. One skilled in the art understands that for different wavelengths, optical fiber materials, and applications, the core diameters and the cladding diameters of a single mode optical fiber and a multimode optical fiber may be different from those of the representative ITU industry standard single mode optical fiber and multimode optical fiber.
The costs of multimode optical components and multimode optical communication systems are generally lower than the costs of the corresponding single mode optical components and single mode optical communication systems. Recently, cost concerns have driven up the use of multimode optical components and multimode communication systems in high data rate communication applications in place of single mode optical components and single mode optical communication systems. One of the high volume optical components in optical communication applications is the optical fiber coupler, particularly the fused optical fiber coupler. Technologies for fabricating a high performance single mode fused optical fiber coupler are understood by many skilled in the art. Fabricating a high performance multimode fused optical fiber coupler, which is suitable for demanding communication applications, however, is a challenge.
There are two major types of fused optical fiber couplers, the surface interaction type and the core interaction type. A representative fabrication method of the core interaction type fused optical fiber coupler includes the step of maintaining the ends of a plurality of optical fibers in contact and fusing the ends of optical fibers together. In a core interaction type fused optical fiber coupler, light propagates from a core end of an optical fiber to a core end of another optical fiber through butt coupling. Core interaction type fused optical fiber couplers are not related to the present invention.
The optical fibers in a surface interaction type fused optical fiber coupler primarily couple through the sides of the optical fibers. Selected side surfaces of the optical fibers are placed in close proximity and fused. The present invention relates to a surface interaction type fused optical fiber coupler. A representative conventional surface interaction type multimode optical fiber coupler is the fused biconical taper multimode optical fiber coupler. The fused biconical taper multimode optical fiber coupler is fabricated according to the fused biconical tapering method. A representative fused biconical tapering method comprises the steps of: twisting a section of a first multimode optical fiber with a section of a second multimode optical fiber and setting up to monitor the optical characteristic of the multimode optical fibers; heating at least a portion of the twisted section to form a fused section and tapering the fused section by pulling the two multimode optical fibers from both sides of the fused section to elongate the fused section until a predetermined optical characteristic is obtained or a predetermined end condition is reached. According to the fused biconical tapering method, a high degree of tapering is important to the fabrication of a high performance optical fiber coupler. Tapering promotes the escape of light propagating in the core of an optical fiber to the cladding and the conversion of light propagating in the cladding of an optical fiber to light propagating in the core. As a result of a high degree of tapering, the cross-sectional area of the fused section of a fused biconical taper optical fiber coupler is typically much smaller than the sum of the cross-sectional areas of the optical fibers. The heating is typically accomplished with an oxyhydrogen flame.
FIG. 1 shows the configuration of a representative conventional fused biconical taper multimode optical fiber coupler. Referring to FIG. 1, the representative conventional fused biconical taper multimode optical fiber coupler comprises a first multimode optical fiber 1 and a second multimode optical fiber 2. First multimode optical fiber 1 and second multimode optical fiber 2 share a fused section 3. Section X–X′ is a representative cross-sectional view of first multimode optical fiber 1 and second multimode optical fiber 2. First multimode optical fiber 1 has a first core 11 and a first cladding 12. Second multimode optical fiber 2 has a second core 21 and a second cladding 22. Section Y–Y′ is a representative cross-sectional view of fused section 3. The total cross-sectional area at section X–X′ is the sum of the cross-sectional areas of first multimode optical fiber 1 and second multimode optical fiber 2. The cross-sectional area at section Y–Y′ is the cross-sectional area of fused section 3. The cross-sectional area at section Y–Y′ is much smaller than the total cross-sectional area at sectional X–X′ because of the high degree of tapering of fused section 3 during fabrication. For many representative conventional fused biconical taper multimode optical fiber couplers, the cross-sectional area at section Y–Y′ is typically about ten percent of the total cross-sectional area at sectional X–X′. While test data indicate that fused biconical taper single mode optical fiber couplers enjoy superb performance, test data show that the representative conventional fused biconical taper multimode optical fiber couplers are less than desirable in some demanding applications.
There are numerous technical challenges in fabricating a multimode optical fiber coupler. One of the technical challenges that is unique to fabricating a multimode optical fiber coupler and have no equivalence in fabricating a single mode optical fiber coupler is overcoming mode sensitivity with little added insertion loss. Many multimode optical fiber couplers exhibit mode sensitivity in key optical parameters, including, for example, insertion loss and coupling ratio. Consequently, the optical parameters of a multimode optical fiber coupler may depend on the mode distribution profile of the multimode light source that provides the light propagating in the multimode optical fiber coupler and the launch method for launching light from the multimode light source into the multimode optical fiber coupler. Therefore, mode sensitivity in a multimode optical fiber coupler is undesirable for many applications. An approach for reducing the mode sensitivity of a multimode optical fiber coupler is to over-fuse the fused section and form an over-fused multimode optical fiber coupler. In an over-fused multimode optical fiber coupler, the cores of the optical fibers are very close together or fused together in the over-fused fused section. Unfortunately, the over-fused fused sections of many representative conventional over-fused fused biconical taper multimode optical fiber couplers are highly tapered. Highly tapered fused biconical taper multimode optical fiber couplers are likely to exhibit high insertion loss.
The second technical challenge is fabricating a mode-insensitive or reduced-mode-sensitivity multimode optical fiber coupler with uneven optical coupling between its optical fibers. Many representative over-fused fused biconical taper multimode optical fiber couplers have even optical coupling, that is, 50/50 coupling ratio for a 2×2 multimode optical fiber coupler. Many applications, however, demand multimode optical fiber couplers with uneven optical coupling, including, for example, 60/40, 70/30, 80/20, 90/10, 95/5, and 99/1 coupling ratios for a 2×2 multimode optical fiber coupler.